Building with GCC 7.1 fails due to the newly-added implicit-fallthrough feature. Some case statements in libcss's parser have a "Fall through" comment but are inside an additional set of braces, which prevents GCC's comment detection working and triggers a warning, which causes the build to fail. An example of this is seen at http://git.netsurf-browser.org/libcss.git/tree/src/parse/parse.c#n2094.